Three-dimensional shaped bodies having a leather-like surface

ABSTRACT

The invention relates to a method for producing three-dimensional shaped skins having a leather-like surface, and to shaped bodies that can be obtained according to this method.

The invention relates to a method for preparing three-dimensionalcasting skins having a leather-like surface, and to the molded partsobtainable by such method.

The use of natural fibers in the automotive industry is steadilyincreasing. The main uses include rigid trim parts in the interior spaceand trunk. To date, vegetable fibers have been exclusively employed asthe fibers. Fibers of animal origin, such as wool, silk or just leatherfibers, have hardly been used to date, except for leather, of course.

In contrast to artificial leathers, which mostly consist of a PVC orpolyurethane layer on a support woven or non-woven fabric, the maincomponent of leather-fiber textile is leather fiber milled from leatherscraps. It is held together by polymeric binders, such as natural latex,polyacrylates, polyvinyl acetates and their copolymers and variousadditives. The properties of the textile are determined by the kind ofproduction, the density and, above all, by the components, i.e., leatherfiber and binder. Since the effect of the leather fiber is quiteprominent, it may be said that leather-fiber textile not only looks likeleather, but also smells and feels like it. In contrast to leather, itsproperties are more uniform due to the usual production as sheeting.

To the end of the 1950's, leather-fiber textile was prepared byhandicraft in so-called screen frames, but in the 1960's and 1970's,most plants converted to continuous production on modified endless-wiremachines, as known from the paper industry.

Further optimizations of formulations and technology enabled a broadrange of thicknesses and flexibilities to be covered, from paper-thin,namely 0.3 mm, to plywood-thick, 6 mm. Depending on its application, theleather-fiber textile either can be creased, like some leathers, or itis rigid and firm, very much like press-board. To date, this has alloweda field of application ranging from calendar and book covers to shoeheels or even the field of automobile interior trims.

The use of leather scraps is known from “Ullmanns Enzyklopädie dertechnischen Chemie”, 4. ed., vol. 16, p. 174. Thus, leather scraps canbe defibrated and subsequently processed to leather fiber materials.These are single-layered sheet materials of leather fibers and binders.The leather fiber materials are then used for bag leather goods, butmainly in the production of shoes and for technical leather gaskets. Forinstance, the leather scraps are defibrated wet or dry in crushingmills, or defibrated wet in toothed disk mills and refiners, or inhollander beaters, wherein the fibers should have a length of from 0.1to 15 mm. Water insoluble, particularly natural, or synthetic rubberlatices as well as dispersions of acrylic ester, vinyl ester andisobutylene polymerizates and mixed polymerizates have proven useful asbinders. The amount of binder is between 8 and 40%. In leather fibermaterials having higher proportions (20 to 30%) of water-insolublebinders, the characteristics of the binder are thus predominant. Inproducts having a lower content of binder (less than 20%), the fibrouscharacter is predominant; such materials are more absorptive and moreleather-like.

From DE 34 17 369 C2, a process is known for producing aninjection-moldable composite material in which a polyester-cotton mixedfiber from waste fabric is fused with a polyolefin. However, thematerial thus prepared lacks both a sufficient water-absorbing capacityand the feel characteristics typical of leather.

DE 21 20 149 A1 describes weather-resistant and torsion-free plates,tubes, rods and other molded articles consisting of binders and fillersmade of used material, including waste from paper, cardboard articles,knitted goods, cottons, linens, synthetic fibers, leather, rags, hay,straw, foliage, grass, shells of cereals and fruits, pits and peels offruits and potatoes as well as metal chips, grains, powders, metals,polystyrene, waste from plastic processing, natural fibers such as jute,sisal or hemp. Information about the amounts of filler materialsemployed is not included.

WO 94/02300 describes molded parts having leather-like surfaceproperties in the automotive field, comprising a thermoplastic orelastic thermosetting material and milled leather scraps in an amount ofup to 95% by weight, based on the molded part.

In contrast, the object of the present invention is to provide a methodfor immediately preparing three-dimensional molded parts from leatherfiber materials.

The above object is achieved by a method for the preparation ofthree-dimensional casting skins having a leather-like surface,characterized in that the porous surface of a vacuum tool having thegeometry of the three-dimensional molded part is introduced into a pulpwhich contains leather fibers, suspending agents and binders, leatherfibers are deposited in the desired layer thickness from the pulp on thesurface of the vacuum tool by applying a vacuum in the vacuum tool,followed by transferring the surface of the vacuum tool to a press toolto densify the leather fiber layer, optionally followed by surfaceprofiling and partial or complete drying, and providing the leatherfiber surface with a finish.

The main raw material is the leather fibers. They are primarilyclassified according to the kind of tanning. The main tanning methodsare chrome tanning (wet blue) and vegetable tanning, and recently alsoglutardialdehyde tanning (wet white). Chrome-tanned fibers are thelargest fraction. They are obtained, for example, from the tannery inthe form of chrome shavings.

A cow hide has a non-uniform thickness, mostly from 4 to 5 mm, and thedesired leather is to have a thickness of mostly around 1 to 1.5 mm.Thus, it is desired to cleave the hide into several layers and then toprovide them with equal thicknesses by means of rotating cutters.Without tanning, this is not possible, because the hide is too supple.After full tanning, it is too expensive because a major portion of thechemicals employed for tanning would be lost together with the shavings.

Therefore, the hide is subjected to preliminary tanning prior to beingshaved, to obtain a leather precursor, the wet blue and the shavings.This means that the shavings must be tanned, greased and dyed by analogywith the tanning process. Since chrome-tanned leathers are particularlysoft and flexible in general, it is also possible to prepare aparticularly soft leather fiber textile from chrome shavings.

Leather scraps from vegetable tanning are predominantly obtained in theform of gratings left from the punching of shoe soles. Such leathers arefully tanned, very firm, but also very rigid. For parts which must havea high dimensional stability, it is the ideal raw material. In contrast,where suppleness is desired, higher formulation demands are required.Nevertheless, vegetable tanning is very interesting, in particular, forautomobile interior trims, because, in contrast to chrome tanning, itenables lower shrinkage when subjected to high temperatures. Inaddition, embossability is clearly better than that of leather fibertextile from chrome-tanned scraps.

Glutardialdehyde tanning is mostly applied in combination with othertanning methods and yields chrome-free leathers having a goodheat-shrinkage behavior. The respective shavings are tanned even moreweakly as compared to the chrome shavings. The preparation of leatherfiber textile from wet white shavings is relatively tedious, and thesuppleness to which we are used from chrome shavings could not beachieved to date.

Although mixing with non-collagenous fibers, such as cellulose, cottonor plastic fibers, for example, polyamide fibers, can cause advantages,such as an increase in production speed or the improvement of mechanicalproperties, these fibers, which are mostly longer, often cause problemsrelating to the surface smoothness after coating. In addition, theleather characteristics are greatly reduced as the content of foreignfibers increases. Nevertheless, the present invention also comprises theuse of such foreign fibers.

The production of leather fiber textile is similar to that of paper. Forexample, the leather scraps are precomminuted dry in a cutting mill,followed by wet milling to the desired fiber length, preferably from 0.1to 15 mm, especially from 0.2 to 3 mm. At this time, the dyes andgreasing agents are also added. The greasing agent is of particularimportance. The fat-water emulsion must be stabilized in such a way thatthe fat completely deposits on the leather fiber and intrudes in itscapillary spaces. The greasing agent also determines the degree offlocculation. If the fibers flocculate too finely, although an excellentmolded part and thus an excellent surface is obtained, the leather fibertextile becomes too hard. Too coarse a flocculation results in a goodsmoothness, but in too little fiber coherence and thus a poor firmness.Of course, the greasing agent should be low fogging.

The binder in the pulp to be employed according to the inventionpreferably consists of thermoplastic and/or thermosetting material andis selected, in particular, from natural rubber, polyurethanes,polyacrylates, dispersions of acrylic esters, vinyl esters andisobutylene polymers and mixed polymers, or a vinyl acetate. The pulpcontains the above mentioned binder, for example, in an amount of from10 to 50% by weight, especially in an amount of from 15 to 30% byweight.

The binder is preferably added as a latex. Latex consists of tinypolymer particles which are suspended in water. In order to keep such asuspension stable, the latex particles mostly have an electric charge ontheir surface. Thus, they repel each other; agglomeration andprecipitation is prevented. If the leather fibers have a charge oppositeto that of the latex particles, these will coagulate directly on thefiber. By optimally adjusting the charge proportions, it is possible toemploy very large amounts of binders without putting a load on the wastewater processing plant.

Into the thus obtained pulp, the porous surface of a vacuum tool, whichmay have both positive and negative shape, having the geometry of thethree-dimensional molded part is introduced. On the surface of thevacuum tool, the leather fibers deposit in a desired layer thickness. Assoon as the desired amount of leather fibers has been deposited on thesurface of the vacuum tool, the vacuum tool with the leather fibersadhering to its surface is transferred to a press tool, and the leatherfiber layer is dewatered and densified, optionally surface-profiled, andduring this or in a separate step, it is optionally subjected to partialor complete drying and provided with a finish. Subsequently, the surfacemay be smoothed by glazing and grinding.

Dewatering and drying are significantly more difficult as compared topaper production. On the one hand, the leather fiber releases the watermuch more slowly, and on the other hand, leather fibers must be dried atvery much lower temperatures. This is because leather will contract atthe so-celled shrinkage temperature and subsequently hornify on thesurface due to hydrolytic processes.

In the finishing step, the leather fiber textile is provided with thesame appearance as that of leather. The coating, coloring, printing andembossing are preferably effected with the same formulations and, inpart, by the same methods as for leather.

According to the present invention, it is particularly preferred toemploy the porous surface of a sintered powder metal, a ceramic, a metalfoam, a plastic foam or a screen for preparing the molded part. Thisporous surface has the geometry of the three-dimensional molded part,for example, an armrest or dashboard of a motor vehicle.

Although leather fibers in any suspension media may be employed inprinciple, it is particularly preferred according to the invention toemploy an aqueous pulp.

It is particularly preferred according to the invention to employ a pulpwhich contains leather fibers in an amount of from 0.1 to 10% by weight,especially in an amount of from 0.5 to 2% by weight.

The desired surface properties of the molded parts to be prepared can beachieved by per se known methods of embossing, grinding, plasmatreatment, corona treatment, sand blasting or shot blasting.

In the method according to the invention, it is particularly preferredto apply the leather fibers in a dry layer thickness of from 0.1 to 6mm, especially from 0.1 to 2 mm, more especially from 0.3 to 0.6 mm. Tothe skilled person, it is obvious that the leather fibers must beapplied wet from the leather fiber pulp at a higher density because theywill shrink to some extent after drying. The term “dry layer thickness”naturally relates to those molded parts which still have a low residualmoisture content of, for example, from 15 to 30% after having beendried, for example, at 70° C. for 2 minutes.

During the drying step, the binder forms a film. This involvespolymerization, polycondensation and/or cross-linking of the binder.Naturally, this also causes mutual cross-linking of the collagen fibersand any foreign fibers through the binder.

To prepare molded parts having a complicated geometry, it is possible toemploy a mold with mobile slides for forming undercuts. This is atechnique which is usual, in particular, in the automobile field for thepreparation of corresponding molded parts from thermoplastic and/orthermosetting materials.

After the molding and, in particular, after release from the mold, it ispossible, for example, to provide the molded part with a foam backing orinjection-molded backing.

The finish can be applied to the dried leather fiber layer in a per seknown manner. This may be done, for example, by coating or spraying ontothe leather fiber layer after drying, and/or onto the surface of themold prior to densifying.

By means of the present invention, it is possible to prepare a number ofthree-dimensional molded parts having a leather-like surface.

According to the present invention, it is particularly preferred thatthe molded parts having a leather-like surface comprise furniture,clothing, accessories, installation parts, veneers and trims, especiallyfor the automotive field. In the automotive field, suitable itemsinclude, in particular, floor trims, trunk trims, roof trims, dashboardtrims, switches, gearshift levers, doorknobs and/or steering wheelcovers, as well as seat cushions, seat rests or rear shelves etc.

In practice, the leather fibers can be employed for any purpose in whichthe optical properties play an essential role. In addition, asound-insulating effect is also important, however.

APPLICATION EXAMPLES Example 1

(a) Preparation of the Leather Fiber Pulp

500 g of aqueous leather fiber pulps with leather fibers with an averagefiber length of 1 mm and a solid content of 2% as well as an addition of20% fat, based on the dry fiber, was admixed with usual tanning agentand aluminum sulfate. This pulp was added to a 1000 ml beaker. To this,30 g of commercially available natural latex with 3% solids content wasadded. Immediately after the latex addition, mixing was performed for 2minutes.

(b) Preparation of the Molded Part

A model of a dashboard was immersed into the leather fiber pulp preparedaccording to (a). The model of the dashboard consisted of a porousceramic body with an average pore size of 0.02 mm. The surface which wasnot immersed in the pulp was not porous. By sucking the ceramic bodyfrom the back, part of the leather fibers deposited on the surface ofthe ceramic molded part.

The molded part was withdrawn from the leather fiber pulp, maintaining areduced pressure.

Then, in a further process step, the dashboard model was transferredinto a mold, and the deposited leather fiber coating was dried by vacuumsucking.

Subsequently, the exposed surface of the leather fibers was providedwith a finish of PU or other materials known from the leather industryand pressed in a per se known manner.

1-17. (canceled)
 18. A method for the preparation of three-dimensionalcasting skins having a leather-like surface, comprising the steps of:applying a pulp comprising leather fibers, suspending agents, bindersand optionally additives, to the porous surface of a vacuum tool havingthe geometry of the three-dimensional molded part; applying a vacuum inthe vacuum tool to deposit said pulp to a desired layer thickness alongsaid porous surface to form a casting skin; and transferring the castingskin to a press tool and applying pressure to remove moisture anddensify the casting skin.
 19. The method according to claim 18, furthercharacterized in that said casting skin is profiled.
 20. The methodaccording to claim 18, further characterized in that said casting skinis dried.
 21. The Method according to claim 18, further characterized inthat said casting skin is provided with a surface finish.
 22. The methodaccording to claim 18, characterized in that the porous surface of saidvacuum tool if formed from a material selected from the group consistingof a sintered powder metal, a ceramic, a metal foam, and a plastic foamor screen.
 23. The method according to claim 18, characterized in thatsaid pulp contains leather fibers in an amount of from 0.1 to 10% byweight.
 24. The method according to claim 18, characterized in that saidpulp contains leather fibers in an amount of from 0.5 to 2% by weight.25. The method according to claim 18, characterized in that the surfaceproperties of the casting skin can be modified by embossing, grinding,plasma treatment, corona treatment, sand blasting or shot blasting. 26.The method according to claim 18, characterized in that a pulp isemployed which contains leather fibers of a length of from 0.1 to 15 mm.27. The method according to claim 18, characterized in that a pulp isemployed which contains leather fibers of a length of from 0.3 to 3 mm.28. The method according to claim 18 characterized in that said binderis selected from the group consisting of natural rubber, polyurethane,polyacrylates, dispersions of acrylic esters, vinyl esters andisobutylene polymers and mixed polymers, or a vinyl acetate.
 29. Themethod according to claim 18, characterized in that said binder ispresent in an amount of from 10 to 50% by weight, based on the dryweight.
 30. The method according to claim 18, characterized in that saidbinder is present in an amount of from 15 to 30% by weight, based on thedry weight.
 31. The method according to claim 18, characterized in thatthe casting skin has an average dry layer thickness of from 0.1 to 6 mm.32. The method according to claim 18, characterized in that the castingskin has an average dry layer thickness of from 0.1 to 2 mm.
 33. Themethod according to claim 20, characterized in that the drying stepcomprises the polymerization, polycondensation, cross-linking and/orfilm forming of the binder.
 34. The method according to claim 18,characterized in that a mold with mobile slides for forming undercuts isemployed.
 35. The method according to claim 18, characterized in thatthe casting skin is released from the surface of the vacuum tool andprovided with a foam backing or injection-molded backing.
 36. The methodaccording to claim 18 characterized in that a pulp is employed whichfurther contains non-collagenous fibers.
 37. The method according toclaim 36 characterized in that said non-collagenous fibers are selectedfrom the group consisting of cellulose, cotton and/or plastic fibers.38. A three-dimensional molded part having a leather-like surface andobtainable by a method according to claim
 18. 39. The molded part havinga leather-like surface according to claim 38, comprising furniture,clothing, accessories, installation parts, veneers and trims.
 40. Themolded part according to claim 39, characterized in that said trim areselected from the group consisting of floor trims, pillar trims, trunktrims, door trims, dashboard trims, switches, gearshift levers, seatcushions, seat rests, doorknobs and steering wheel covers.